Retinoid and steroid hormones regulate a variety of physiological processes from morphogenesis to reproduction. Biosynthesis of both types of hormones involves oxidation and reduction of their corresponding precursors to the biologically active forms. Recent data from this and other laboratories suggest that mammalian tissues contain a group of elated enzymes capable of utilizing in vitro both retinoid and steroid alcohols and aldehydes as substrates. These enzymes share more than 40 percent sequence identity and belong to the superfamily of short-chain dehydrogenases/reductases. The main hypotheses of this proposal are that: (1) this group of structurally related enzymes is involved in biosynthesis of both retinoid and steroid hormones in vivo; (2) individual retinol/sterol dehydrogenases exhibit different tissue-specific expression patterns in human tissues, which determines their contribution to each metabolic pathway; and (3) the active site of these microsomal dehydrogenases faces the cytosol, where the NAD+-dependent enzymes function in the oxidative direction, and the NADP+-dependent enzymes function in the reductive direction. The existence of common enzymes should provide the means for joint regulation of retinoid and steroid signaling pathways. This hypothesis is consistent with the observations that retinoic acid significantly decreases serum levels of dihydrotestosterone and that the levels of retinoic acid are decreased in prostate carcinoma tissue. The overall objective of this proposal is to determine the role of the human microsomal retinol/sterol dehydrogenases, recently identified by this laboratory, in retinoid and steroid metabolism in the cells. The specific aims of this proposal are to: (1) develop a procedure for preparation of catalytically active purified recombinant retinol/sterol dehydrogenases; (2) characterize the substrate specificity and catalytic properties of the human retinol/sterol dehydrogenases; (3) determine the expression patterns of retinol/sterol dehydrogenases in human tissues; and 4) determine the topology of transmembrane insertion of human dehydrogenases in the microsomal membrane. Accomplishment of these objectives will allow us to gain understanding of the role of this new group of short-chain dehydrogenases in retinoid and steroid metabolism in health and disease.